Illegal immigration under the laws of the British mandate for Palestine.
Thats generally how it works. Squatters rights, statute of limitations, etc. Doesn't make it right, just accepted. Also, Jews conquered the land from the Canaanites and such, so under your logic and mine its not their land.
The population numbers are vastly different now, the jews are the vast majority now with mass migration of European jews and such so the point is moot, but the Palestinians still got screwed.